0851996159

The goal of the research was to take as many measures of pathology as

possible, with the expectation that the total picture will reveal some

measure of virulence from both the parasite and host perspectives.

Measuring host mortality

The most difficult data to obtain on virulence may be the most interesting:

reduction in host lifespan. Several methods were used to detect any

increase in mortality induced by Plasmodium infection in lizards.

Naturally infected fence lizards suffer two to six times higher mortality

in laboratory cages and, for all parasite species, infected animals are

more prone to attack by predators (Schall, 1996). Prevalence (percentage

infected) rises with the lizard’s age for every system studied, but

prevalence typically levels off or even drops for the oldest lizards

(Fig. 14.4). Long-term mark–recapture studies of infected fence lizards

(Bromwich and Schall, 1986; Eisen, 2000) and laboratory-held anoles

and rainbow lizards show that infections are seldom eliminated. Thus, the

dip in prevalence suggests that mortality increases for older lizards when

they are infected. Among sites at Hopland and Saba, there is a negative

relationship between maximum body size and the prevalence of malaria

infection (Fig. 14.5). Thus, lizards tend to be smaller in areas where

prevalence of the parasites is highest. Lizards typically grow throughout

298 J.J. Schall

< 80 80 90 100 110 120 > 124

Males

Females

Per cent of lizards infected

15

20

5

10

Fig. 14.4. Prevalence ofPlasmodium giganteumin rainbow lizards,Agama

agama, in Sierra Leone, West Africa. The pattern shown here is typical for lizard

malaria parasites. Prevalence increases with size (= age), as expected if lizards

seldom lose infection. Males are more often infected than females. Prevalence

drops for the oldest animals, suggesting that the parasite increases mortality for

these animals.